Method for the treatment of extruded polytetrafluoroethylene



May 31, 1960 w. L.. sLADE METHOD FOR THE TREATMENT OF EXTRUDED POLYTETRAFLUOROETHYLENE Filed Aug. 18, 1955 R L E ..N. O D wl. N o T A 4 T G 4 O Z N L A N Z 3 E S I G I V u M m 5 N N L O T K N P Z W M m wn s s N W j Y l M B I INERT GAS GENERATOR COOLING WATER SOURCE NIETHOD FOR THE TREATMENT OF EXTRUDED POLYTETRAFLUOROETHYLENE Winton L. Slade, Lancaster, Pa., assignor to Raybestos- Manhattan, Inc., Manheim, Pa., a corporation of New Jersey Filed Aug. 18,1955, Ser. No. 529,236

Claims. (Cl. 18-55) This invention relates to the treatment of extruded polytetrailuoroethylene as it comes device. l

In the extrusion process, as now well understood, the polymer in powdered form is mixed with a lubricant to form a paste-like compound which can be extruded in a suitable device such as a ram-type extruder. In the preferred commercial practice, the compound is extruded vertically downward and `the extrudate passes downward by gravity through a treatment apparatus comprising a relatively low temperature vaporizing zone and a relatively high temperature sinter zone. To remove the lubricant, it has been the practice to subject the extrudate in the vaporizing zone to an upwardly flowing stream ofheated air which serves to vaporize and drive off the lubricant. In the sinter zone, the eXtrudate is subjected to a temperature suciently high to rsinter the polymer into a homogeneous mass.

In carrying out this process, it is generally desirable to effect as complete removal of the lubricant `as possible. Furthermore, the most efficient lubricants are flammable, and it has been necessary to maintain the proportion of lubricant vapor to air below the minimum explosion limit in order to prevent likelihood of re and explosion. For these reasons, it has been necessary to control very carefully the volume and rate of flow of the air through the vaporizing zone. However, despite careful control of the air ilow, the flammable lubricant sometimes catches lire.

One object of the present invention is to overcome these objections and to provide an improved process for the treatment of extruded polytetrauoroethylene.

I have discovered that it is possible to employ a heated inert gas to effect vaporization of the lubricant, thereby eliminating the ire and explosion hazard, and elimfrom the extruding inating the need for very careful control of the gas s? flow. Not only have I found that an inert gas can be efficiently employed as the lubricant-vaporizing medium but I have also found that a simple structural arrangement can be employed which will prevent any air from entering the vaporizing zone.

In the treatment of extruded polytetrauoroethylene, it is desirable to provide a soaking zone immediately following the sinter zone to permit equalization of the temperature throughout the wall of the extrudate and to allow additional time for coalescing of the polymer. Without the use of a soaking zone it is very difficult to attain a uniform sinter temperature and to hold the extrudate at that temperature for additional time. However, in prior usages of a soaking zone employing radiant heat diiculties have been encountered in attaining the required temperature control. I have discovered that these diiiiculties can be overcome by the use of convection heat transfer in the soaking zone, preferably by introducing heated inert gas into this zone as well as the vaporizing zone.

icc

is a vertical sectional view of a preferred form4 of the apparatus employed according to this invention.

In the drawing, there is shown a hollow vertical tower 1, preferably in the form of a long rigid tube or conduit of circular cross section, arranged below the extruding die 2 of an extruding device to receive the extrudate` 3 as it comes from the die. The extrudate moves downwardly by gravity through the vertical treatment tower.

In the course of the downward movement of the ex-l trudate, it moves successively through the vaporizing zone 4 and the sinter zone 5. In the vaporizing zone, the lubricant is vaporized and driven off, while in the sinter zone the extrudate is subjected to heat radiated from the wall of the oven and produced by an electrical heating unit `6 which is surrounded by a thick heat-insulating jacket 7.

As thus far described, the apparatus is generally similar to that previously employed. As hereinbefore mentioned, the preferred lubricants employed in the extruding process are arnmable, one such lubricant being that known as varnish makers and painters naphtha. VIn the past, vaporization of the lubricant has been effected by heated air flowing upwardly through the vaporizing zone, and even though substantially pure air is employed and the proportion of lubricant vapor to air is kept well below the minimum explosion limit, tires can still occur. This comes about because some `of the lubricant vapor is driven into the interior of the tubing by the heat in the vaporizing oven, and this vapor is ,forced by its own vapor pressure to ow down through the tubing `at a rate faster than the rate of extrusion until it escapes from the open end, or if the tubing is a long continuous extrusion, until it condenses to a liquid in the cooler zones below the sinter oven. When the open A further object of this invention, therefore, is to pro; j

end of a new extrusion first enters the sinter oven, 1ubricant vapor escaping from `this end will usually ignite and burn like a. torch. Then again, during an extrusion run, if it should happen that'there are any holes in the wall of the tube, lubricant Vapor escaping from these holes into the sinter oven will likewise ignite. Another possibility is that the rate of extrusion may be accidentally increased, thus releasing large quantities of lubricant vapor into the vaporizing zone in a short period of time, with the result that the minimum explosionlimit is exceeded, causing lire or explosion.

In accordance with the-present invention, these objections are overcome by performing yall heating operations in an atmosphere of inert gas. While this gas,

may be introduced at the lower part of the vaporizing zone, it is preferably introduced through an entrance 8 at the lower part of the treatment tower. The inert gas may be supplied from a conventional gas generator 9. The gas ows upwardly through the sinter zone and through the vaporizing zone to an exit 1t) provided at the top of the tower. The inert gas employed may be nitrogen,jcarbondi oxide, superheated steam, or any gas which does not react with the lubricant vapor at the temperatures involved, or does not react exo-thermally so as to liberate large amounts of heat as in combustion. The gas may alsobe a mixture of inert gases. The preferred gas is a typical flue gas, or products of combustion, resulting from essentially complete burning of natural gas, or

other available fuel, with approximately tlieoreticalair..k

When using natural gas, the products of combustion will Patented May 3l, 1960 in which a soaking zone is 3 be 'aniixtu're of -itrogm Ycarbon dioxide and water vapor. Awith .traces of other gases but virtually no oxygen.

The inert gas is supplied at a temperature of approximately 700 to 750 F. As it passes through the vaporizing zone, its average temperature-is about '400 to 500 F,. and when `it .reaches the exit `10, its temperature is about 100 to 150 F.

For effective useof 'the inert gas, it is necessary lto prevent vair `from entering the bottom of the treatment tower, and in the apparatus shown, this'is accomplished by providing a water trap 11 at the bottom of the appa. ratus through which the extrudedtubing may be passed without permitting air to enter. It is practically .iinpossible to seal the upper end of the tower into which the soft extrudate enters, but .I have found that -the upper end of the tower may be left open to the air, and that this is actually advantageous. The air which enters the open end mixes with the inertv gas and-is drawn through the exit-10.. This air -has a-cooling elect, with the VYresult that the air-gas mixture drawn through vthe exitby exhaust lblower 12.y has a relatively low temperature of about 100 to 150 1F.

.Ashereinbefore indicated, 'the heat Yfor 'sintering .the polymer extrudate in the sinter zone is transferred largely orcntirely by radiation from -the heated walls of Vthe oven. The use of radiant heat transfer in the sinter oven is commercially desirable because highrates of heat transfer, lwith corresponding allowable high rates of extrusion, are obtainable with a relatively small sinter oven occupyinga small vertical height. Such a radianttype sinter oyen, while compact and eicient, is not easily controlled Thus., it is necessary .to .Synchronize-Carefully, thesinter oven wall temperature with the extrusion speed, in order to avoid under-signage 9v: cvQr-Snteringo thelpolymer extrudate. The higherthe sinter oven `radi ant temperature, the faster will be the. properly synchronized legitru'sion, speed -for ideal situering, but the more will beA the synchronization.

Furthermore, it is d 4lrable to provide a soaking zone immediatelyY following the sinter `Zone to. permit equaliiing'of the'temperature throughout'the wall of the extrudate,.and to Vallow Vadditililal time for coalescingrol the'polymer, butfthe reqircd'ltemperature controly in thiszone is very'dicult' to attain iflradiant heat alone isfsupplied.

v*In'accordance with afu'rt'len'fefaft'u're'of'thepresent if'vention, anl arrangement is( provided by which the use of a soaking zone is 'madefpossible'and practical. 'As shown in the illustrated embodime'nna soaking'Zo'ne-IS is`piovided below the `sinter ione,l andthe inertga'sfis supplied at the Vlower part of the' soaking 'zone so that 'it passes upward through' that zone and then lthro'u'ghtthe sinter zone'and the vaporizingzfone. The 'temperature f'theinert gas is Vregulatedl at the gas generator so asl to be slightly above the sintering temperature of'polytetrafluoroethylene and approximately'at the temperature desired in thesoaking zone. As previously mentioned, the gas is preferably supplied'at a Atemperature of approximately 700 to 750 F. The'movement of thelheiated inert gas upward is easily attained byutilizing Athe chimney etect ofthe vertical tower. -While the upwardly owing gas, as it passes through the soaking lzone/and the sinter zone, is at or above thev sintering temperature, Vthe additional radiant heat from a heating unit, such las shown at 6, is ordinarilyrequired in the' sinter zone to promote rapid rise of ythe extrudate 'temperature in this`v zone. Some cooling of the inert gas vin the vaporizing-zone will ordinarily be required, and if necessary cooling-of the ga-s at the lower -part of thev vaporizing zone may be efectdby the usefof a cooling coil 14 to which cooling water may Ibe supplied from a` sour'cclS by a 'pump 16. VSuch coolingV ofthe 'inert-'gas need only be enough to'pre'vent'tooi'apid volatiliaationfbf the lubricant from-the 'extrudateg a condition lwhich may cause bubbling within -the'frnass fthe' XtrlldatG.

A'fter the extruded tubing leaves the soaking zone 13,

itis often desirable ytoeft'ect a sudden, but partial .cooling of the tubing through the temperature range of about 100 F. just below the transition temperature of 621 F., in order to limit the growth of crystallinity in the polymer. Apparatus for achieving this. sudden, partial cooling may consist of a line water spray, a Water mist, a coldgas jet, or other means, and .is shown in the illus.- trte'd'embofdiment-as affine water spray 17, supplied from the cooling water source '1S by pump 18, and located near Athe :top of achamber 19.

Whethersuch j'partial .cooling apparatus is used or not, the tubing descends through chamber 19 where it is required to makea turn in order-to pass through the water trap, and this requires 'that it be maintained at a sutilciently high temperature so as to be flexible enough to make this turn. To insure sufcient flexibility of the tubing, a gas exit Z0 is provided just above the water trap,=so that-@portion of the .hot inert gas will be drawn e moving counter to the heated'inert gas. 55

downward -throughrchamber i19 to .the exit 20 by exhaust blower Z1. This gas keeps the chamber, and the tubing -within the chamber, suliiciently warm so that'the tubing remains flexible Venough to make the turn. Preferably, about ofvthe gasows upwardly from the entrance 8, {While 25% is-drawn downward to the kexit Z0. Thus, inone embodiment of the apparatus, the inert gas A`enters throughcntrance `8 at the rate of 2000 cubic feetper` hourfand the upwardly iowinggas is at the rate of 1500 cubic feet -per hour -while the-downwardly drawn gas is 'at the-.rate of 500 cubic feetjper hour, all gas volumes being measured under the standard conditions andon a dry (basis.

'The 'water trapwshownin vthe illustrated embodiment a simple meanslfor enabling withdrawal ofthe tubing whileexcludingair. The Vsame result can be achieved in other ways, as by ,providing a gasket and causing the exhaust blower `21.to.draw..off such air as leaks through the gasket vIf desired, thetubing could be collected withinfthe bottom chamberon. a reel, and emptied batchwise at the endof a run.

'Considering 'the over-alloperation of the apparatus shown,!the extrudate enters vthe vaporizing zone where itis exposed to'heatcd inertgas at fan average temperatureofapproximately '450 Vligand as ittpa-sses downward through thefrisng heatedv inert. gas, it. loses lubricant by vaporiz'ati-on.V .It.then passes into Vthe .relatively short sinterzone'where it isV exposed to inert gas somewhat in excess o f 700 F., and it also receives radiant heat from the"oven"'wallwhichmay' be at a temperature of about 1l00 V1:". `Principally as aresult of .intense radiation, it is raised`quickly`to approximately 700 F., during its passage through the`sinter oven, Vbut; it may be unevenly heated therein. It then enters "the soaking zone, still In this zone, it receives no 'appreciable` .radiation :from the wall but itis permitted toapproach closelyto the uniform gas temperature vof about 700, a typical desired soaking temperature, :entirely by convection heat transfer. lt finallyV passes through the cooling zone and out through the water trap.

Under nocircumstances can tires or explosions result inithlissystem;sinceL-no oxygen is present. The sintering isacheve'd both rapidly and uniformly and with accurate soaking. Furthermore, largevolumes of inert gas are not'lrequired, asfin thc case oflheated Yair where large volumes-are required to maintain a proportion of air and lubricant vapor below the explosive limit. Since most of'the heat #used'for vaporizing, and a portion of that u's'ed forl'sintering, comes from the combustion of fuel rlatlerS-than` from' electrical energy, substantial operating economy 'ispossible While a'-pr'eferred embodiment of the invention'has beendc'Scribed,-the inventionV -is notl limited thereto-but contemplatessuchmodications and further embodiments as may occur to those skilled in the art.

I claim:

1. A process for making tubing of polytetrauoroethylene which comprises extruding vertically downward by means of a die a tube of a paste-like compound comprising tetraluoroethylene polymer and a volatile, flammable lubricant, passing said tube vertically downward and sequentially through a vaporizing zone, a sintering zone, a soaking zone and a cooling zone, introducing to said soaking zone a heated inert gas at a rate of about 2000 cu. ft. per hour and at a temperature of from about the transition temperature of the polymer to about 750 F. and passing at least a portion of said heated inert gas successively through said sintering zone and said vaporizing zone, vaporizing said lubricant by means of said heated inert gas in said vaporizing zone and removing said inert gas and said vapon'zed lubricant from said vaporizing zone, said inert gas having an average tem# perature of from about 400 to about 500 F. at the point of removal from said vaporizing zone, rapidly heating said tube by means of radiant heat from a radiant surface in said sintering zone to a temperature sufficiently high to sinter said polymer, the temperature of said radiating snr-face being synchronized with the rate of extrusion to avoid over sintering of said polymer, maintaining said tube in said soaking zone for a time to permit equalizng of the temperature through the wall of said tube and coalescing of said polymer, rapidly cooling said tube in said cooling zone to a temperature at least about 100 F. below the transition temperature of said polymer in order to limit growth of crystallinity of said polymer, and removing said tube from said cooling zone.

2. The process of claim l in which a portion of said heated inert gas passes from said soaking zone to said cooling zone from where it is thereafter exhausted.

3. The process of claim 1 in which said tube is cooled in said cooling zone by a line water spray.

4. The process of claim 1 wherein said heated inert gas introduced to said soaking zone is at a temperature of from about 700 to about 750 F., and the average temperature of said inert gas at the point of withdrawal from said vaporizing zone is about 450 F.

5. The process of claim 4 wherein said inert gas cornprises substantially oxygen-free products of combustion of a fuel.

References Cited in the file of this patent UNITED STATES PATENTS 1,950,026 Dreyfus et al. Mar. 6, 1934 1,952,877 Mancini Mar. 27, 1934 2,232,012 Rooney et al Feb. 18, 1941 2,425,782 Bludworth et al. Aug. 19, 1947 2,685,707 Llewellyn et al. Aug. l0, 1954 

1. A PROCESS FOR MAKING TUBING OF POLYTETRAFLUOROETHYLENE WHICH COMPRISES EXTRUDING VERTICALLY DOWNWARD BY MEANS OF A DIE A TUBE OF A PASTE-LIKE COMPOUND COMPRISING TETRAFLUOROETHYLENE POLYMER AND A VOLATILE, FLAMMABLE LUBRICANT, PASSING SAID TUBE VERTICALLY DOWNWARD AND SEQUENTIALLY THROUGH A VAPORIZING ZONE, A SINTERING ZONE, A SOAKING ZONE AND A COOLING ZONE, INTRODUCING TO SAID SOAKING ZONE A HEATED INERT GAS AT A RATE OF ABOUT 2000 CU. FT. PER HOUR AND AT A TEMPERATURE OF FROM ABOUT THE TRANSITION TEMPERATURE OF THE POLYMER TO ABOUT 750*F. AND PASSING AT LEAST A PORTION OF SAID HEATED INERT GAS SUCCESSIVELY THROUGH SAID SINTERING ZONE AND SAID VAPORIZING ZONE, VAPORIZING SAID LUBRICANT BY MEANS OF SAID HEATED INERT GAS IN SAID VAPORIZING ZONE AND REMOVING SAID INERT GAS AND SAID VAPORIZED LUBRICANT FROM SAID VAPORIZING ZONE, SAID INERT GAS HAVING AN AVERAGE TEMPERATURE OF FROM ABOUT 400* TO ABOUT 500*F. AT THE POINT 